The present invention relates to papaya plants and plant parts having a mutant allele designated EWSMHP, which confers production of highly hermaphroditic progenies upon selfing of its hermaphrodite plants and also production of highly hermaphroditic F1 progenies when crossed with normal female and normal hermaphrodite papaya plants. Further, the present invention relates to papaya cultivar EWS-2499m having mutant allele EWSMHP. All publications cited in this application are herein incorporated by reference.
There are numerous steps in the development of any novel, desirable plant germplasm. Plant breeding begins with the analysis and definition of problems and weaknesses of the current germplasm, the establishment of program goals, and the definition of specific breeding objectives. The next step is selection of germplasm that possesses the traits to meet the program goals. The goal is to combine in a single variety an improved combination of desirable traits from the parental germplasm. These important traits may include but are not limited to higher fruit yield, seed yield, resistance to diseases and insects, better stems and roots, tolerance to drought and heat, altered fatty acid profile, abiotic stress tolerance, improvements in compositional traits, and better agronomic quality.
These processes, which lead to the final step of marketing and distribution, can take from eight to fifteen years in papaya over a sufficient number of generations and with careful attention to uniformity and selection of plant type and traits. Therefore, development of new cultivars is a time-consuming process that requires precise forward planning, efficient use of resources, a minimum of changes in direction, and time for the self-selection of uniformity and traits over numerous generations.
Papaya (Carica papaya L.) is an important fruit tree commodity in many tropical and sub-tropical countries, including many countries in Southeast Asia (Thailand, Philippines, Vietnam, Indonesia) and South Asia (India, Sri Lanka, Bangladesh), Central America (Brazil, Mexico, Ecuador, Colombia) and Africa (Nigeria, Ethiopia, Kenya). Papaya, however, is believed to be native of tropical America (Morton, 1987). Papaya thrives best in areas with a temperature range of 21-33° C. and grows best in light, well-drained soils rich in organic matter (Prosea).
Papaya has a very unique sex expression. Papaya is a polygamous plant species with generally three (3) primary sex types: 1) staminate (male); 2. hermaphrodite (bisexual); and 3. pistillate (female). The male flowers are characterized by long, pendulous, freely-branching inflorescenses consisting of either exclusively or predominantly staminate flowers. Female trees have short inflorescences usually consisting of only five or six flowers that are pistillate exclusively, 3-5.5 cm long with yellowish green cup-shaped calyx. Hermaphrodite trees have relatively short, few-flowered inflorescences consisting mainly of bisexual flowers.
In many countries, papaya growers prefer to grow only the hermaphrodite plants since hermaphrodite plants produce long cylindrical fruits which are preferred by traders and consumers. Female plants produce mostly round fruits with bigger seed cavity and hence, less ideal for transport due to its rounder shape. Most papaya growers plant 3 or more seedlings per hill and during flower initiation, all female trees are cut leaving only 1 hermaphrodite plant per hill. This system requires more seeds and since 3 or more seedlings are grown initially, competition takes place and sometimes resulting in non-uniform size of trees in the later stages of crop development. Hence, a papaya variety that can produce high proportion of hermaphrodite trees will be preferred by papaya growers. The identification of a true-breeding hermaphrodite papaya can revolutionize papaya production since it will completely eliminate the planting for 3 or more plants per hill as currently practiced in countries that prefer only hermaphrodite plants.
Papaya has three distinct chromosomes: Y for males, Yh for hermaphrodites and X for females. Storey (1938) has illustrated the different segregation ratios observed in papaya. For example, selfing the hermaphrodite plants will result in a segregation of 67% hermaphrodites and 33% females. Pollinating a female flower with pollen from a hermaphrodite flower will result in progenies with a segregation of 50% females and 50% hermaphrodites. Pollinating a female flower with pollen from a male plant will result in progenies with a segregation of 50% females and 50% males. These intriguing ratios led to the hypothesis that there is a lethality gene that is linked to the Y and Yh chromosomes (Storey, 1953; 1969), such that any combination of YY, YhY and YhYh genotypes will be lethal. Therefore, all male and hermaphrodite papaya are enforced sex heterozygotes (male=XY and hermaphrodite—XYh). The segregation ratios are further illustrated in Tables 1-3 below. Table 1 shows the segregation ratios when a female papaya is crossed with a male papaya. Table 2 shows the segregation ratios when a female papaya is crossed with a hermaphrodite (herma) papaya. Table 3 shows the segregation ratios when a hermaphrodite (herma) is crossed with a hermaphrodite papaya.
TABLE 1Female × MaleMale (XY)Female (XX)XYXXX (F)XY (M)XXX (F)XY (M)Ratio:½ F½ M50% F50% M
TABLE 2Female × HermaHerma (XYh)Female (XX)XYhXXX (F)XYh (H)XXX (F)XYh (H)Ratio:½ F½ H50% F50% H
TABLE 3Herma × HermaHerma (XYh)Herma (XYh)XYhXXX (F)XYh (H)YhXYh (H)YhYh (lethal)Ratio:⅓ F⅔ H33% F67% H
Because of the need to detect the sex of the papaya plant early, the identification or development of markers associated with papaya sex expression was a goal for some researchers. In the 1930's, Hofmeyr (1939) discovered two morphological markers—flower color and stem color, which were linked to the sex determination locus. These two morphological markers, however, were about 24 and 40 cM away and provided very little predictive value.
The development of molecular-based techniques led to the use of PCR-based markers associated with the sex of papaya. In the 1990s, more research was done to develop/identify sex-linked DNA markers for determining papaya sex types. The first sex-linked DNA marker reported was a microsatellite containing the (GATA)4 repeat (Parasnis et al., 1999). Using randomly amplified polymorphic DNA (RAPD) markers, four sequence-characterized amplified region (SCAR) markers were developed by other research groups to predict sex-types (Parasnis et al., 2000; Urasaki et al., 2002; Deputy et al 2002).
While markers provide an early detection method to determine the sex of the papaya plant at the very early seedling stage, the cost of running these markers routinely is high, significantly more expensive than the cost of the seed itself. Others have used in vitro technology by cloning and asexually propagating hermaphrodite plants and selling in vitro-derived papaya seedlings. This method ensures the planting of 100% hermaphrodite plants, but the cost of in vitro-derived papaya seedlings is very high and requires dedicated facility and equipment for mass propagation of hermaphrodite papaya plants.
Hence, a method of producing high proportion of hermaphrodite plants without using markers for sex genotyping or cloning in vitro hermaphrodite plants would be highly desirable.
The foregoing examples of the related art and limitations related therewith are intended to be illustrative and not exclusive. Other limitations of the related art will become apparent to those of skill in the art upon a reading of the specification.